The invention relates to a pump for a high-pressure cleaning device for delivering a cleaning fluid, comprising at least one pump chamber, into which at least one piston plunges which can be moved back and forth and which is connected to a suction line via at least one inlet valve and to a pressure line via at least one outlet valve, and comprising a bypass line which leads from the pressure line to the suction line and in which an overflow valve is arranged, the valve body of which is connected to a control piston which is displaceably held in a control chamber with the interposition of a sealing element and which moves the valve body into a closed position or an open position as a function of the flow rate of the cleaning fluid in the pressure line, wherein the control piston divides the control chamber into a high pressure chamber which is connected to the pressure line upstream of a flow restriction point and a low pressure chamber which is connected to the pressure line downstream of the flow restriction point.
Pumps of this type are known from DE 196 07 881 A1. They can be used to subject a cleaning fluid, for example water, to pressure and to subsequently direct it at an object via, for example, a pressure hose which can be connected to the pressure line and a nozzle head which is arranged at the free end of the pressure hose. So that the mechanical load on the pump as well as heat losses can be reduced, the cleaning fluid delivered by the pump is guided in a circuit with as little flow resistance as possible when the nozzle head is closed, i.e. it will be returned from the pressure line to the suction line again so that the pressure in the pressure line sinks. For this purpose, the pressure line is connected to the suction line via a bypass line and an overflow valve is arranged in the bypass line. During working operation of the pump, i.e. when the nozzle head is open, the overflow valve closes the flow connection between the pressure line and the suction line. If the nozzle head is closed, the overflow valve will release the flow connection between the pressure line and the suction line so that the pressure prevailing in the pressure line is reduced. The valve body of the overflow valve is connected for this purpose to a control piston which moves the valve body into a closed position or an open position as a function of the flow rate of the cleaning fluid in the pressure line. The flow rate of the cleaning fluid in the pressure line is dependent on whether the nozzle head is open or closed. If the nozzle head is closed, the flow rate drops and, as a result, causes the control piston to move the valve body of the overflow valve into its open position so that the cleaning fluid subject to pressure can flow to the suction line with as little flow resistance as possible. If the nozzle head is open, the flow rate in the pressure line increases and this causes the control piston to move the valve body of the overflow valve into a closed position and so the pump transfers into the normal operating state.
The control piston is displaceably held in the control chamber and divides this into a high pressure chamber and a low pressure chamber. The high pressure chamber is connected to the pressure line upstream of a flow restriction point and the low pressure chamber is connected to the pressure line downstream of the flow restriction point. An injector can, for example, be arranged in the pressure line as flow restriction point and with its aid a cleaning chemical can be mixed into the cleaning fluid subject to pressure. When a flow of fluid is present in the pressure line, the flow restriction point results in the pressure downstream of the flow restriction point differing from the pressure upstream of the flow restriction point. Since the high pressure chamber is connected to the pressure line upstream of the flow restriction point, whereas the low pressure chamber is in communication with the pressure line downstream of the flow restriction point, the control piston is acted upon with a differential pressure when a flow of fluid is present in the pressure line. On account of the differential pressure acting on it, the control piston moves the valve body of the overflow valve contrary to the direction of flow prevailing in the bypass line into a closed position, in which the valve body abuts on a valve seat of the overflow valve. If the flow of fluid in the pressure line is interrupted, the flow restriction point does not cause any drop in pressure and the pressure in the low pressure chamber corresponds to the pressure in the high pressure chamber. When a difference in pressure between the two chambers is not present, the control piston can be acted upon with a resulting force which is dependent on the areas exposed to pressure of the two chambers and as a result of which it is moved in the control chamber in such a manner that the valve body transfers into its open position. The valve body is then at a distance from the valve seat and, as a result, the flow connection between the pressure line and the suction line is released for a circular flow operation of the pump.
The control piston effects a sealed separation of the high pressure chamber from the low pressure chamber. For this purpose, a sealing element is arranged between the control piston and the control chamber. During the working operation of the pump, a considerable difference in pressure acts on the sealing element. The sealing element must reliably withstand this difference in pressure even after longer operation of the pump since a leaky separation of the low pressure chamber from the high pressure chamber in the region of the control piston can lead to impairment of the pump.
The object of the present invention is to further develop a pump of the type described at the outset in such a manner that it is less susceptible to malfunctions and more inexpensive to produce.